Biodiesel, which is derived from vegetable oils or animal fats through transesterification to produce mono-alkyl esters, exhibits fuel properties comparable with conventional petroleum diesel. Biodiesel provides enhanced lubricity properties and produces low exhaust emissions, such as particulate matter, polycyclic aromatic hydrocarbons, carbon dioxide, sulfur dioxide and smoke. However, a significant problem associated with the commercial acceptance of biodiesel is poor oxidative stability.
The presence of high levels of unsaturated fatty acid methyl esters (FAME) makes biodiesel very susceptible to oxidation as compared to petroleum diesel. Oxidative processes bring about increased viscosity as a result of condensation reactions involving double bonds, also leading to the formation of insolubles, which can potentially plug fuel filters and injection systems. The increased acidity and increased peroxide value as a result of oxidation reactions can also cause the corrosion of fuel system components, hardening of rubber components, and fusion of moving components. ASTM D6751-07 includes an oxidation stability standard of a 3 hour minimum induction period (IP) as measured using the Rancimat test (EN14112). The European Committee for standardization adopted a 6 hour minimum IP as the specification. A survey of retail biodiesel samples performed in 2004 indicated that only 4 out of 27 B100 samples met the oxidative stability standard of 3 hour and over 85% had an IP less than 2 hours. In a 2006 survey report, the range of induction periods in 10 samples was 0.43 to 4.26 hours, and only 3 out of 10 B100 samples met the standard. Our survey of B20, B10, and B5 samples from retail stations also found that over 50% had an IP less than 6 hours, the proposed ASTM oxidative stability for B6-B20.
Factors which influence the oxidative stability of biodiesel include fatty acid composition, natural antioxidant content, the level of total glycerin, and the conditions of fuel storage such as temperature, exposure to light and air, and material of tank construction. Previous studies have found that antioxidants can be effective in increasing the stability of biodiesel. However, these effects have not been fully elucidated and results have been inconclusive or conflicting. Butylated hydroxyanisole (BHA) and butyl-4-hydroxytoluene (BHT) have been shown to have nearly the same effect on the oxidative stability of rapeseed oil- and tallow-based biodiesel, and the optimal level of synthetic antioxidants was determined to be 400 ppm. It has been reported that pyrogallol (PY), propylgallate (PG), and t-butylhydroquinone (TBHQ) could significantly improve the stability of biodiesel obtained from rapeseed oil, used frying oil, and beef tallow, whereas BHT was not very effective. Moreover, it has been found that BHT had the highest effectiveness for refined soybean oil-based biodiesel, while BHA displayed little effectiveness.
The major feedstock for biodiesel production is rapeseed oil in Europe, while soybean oil is the major feedstock in the USA. Biodiesel made from soybean oil has a significantly higher content of methyl linoleate (C18:2) and methyl linolenate (C18:3) than that made from rapeseed oil, and therefore soy-based biodiesel demonstrates noticeably poorer oxidative stability. Moreover, in a stability study of biodiesel and biodiesel blends, long-term storage of biodiesel was recognized as an important issue. Although the BIOSTAB project conducted in Europe focused on the long-term stability of rapeseed-based biodiesel at room temperature, and outside ambient temperature for up to 24 months, few studies have evaluated soy-based biodiesel.
A few studies have revealed that antioxidants improve the oxidative stability of biodiesel, and reports have been made about the synergism of antioxidants in edible oils and fats and in lubricating oils, however the synergy of synthetic antioxidants in biodiesel has not been fully elucidated.
Because commercial acceptance of biodiesel has been limited by poor oxidative stability there is need develop new biodiesel formulations having increased oxidative stability for commercial use.